This invention relates to fired heaters and, more particularly, to fired heaters for heating a multiphase feedstock.
Fired heaters employing a plurality of tubes for passing a feedstock through a plurality of passes to heat the feedstock are well known. In these designs, the tubes are often bent back several times in a serpentine relationship to reverse the direction of the feedstock flow.
However, when dealing with a three-phase feedstock, such as a mixture of coal, oil and gas in slurry form such as found in an oil refinery, a coal treating plant, or the like, several problems are encountered by virtue of the existence of the solid coal suspended in and carried by the liquid and gas. For example, a major problem stems from the fact that the solid feedstock, which is naturally abrasive and which often is of a relatively high volume and at a relatively high pressure flowing at high velocities, impinges on the walls of the tubes and therefore causes erosion. Since the above-mentioned bends in the tubes extend for as much as 180.degree., this impingement, and resulting erosion, is especially severe in these bent areas.
One possible solution to this problem is to eliminate the conventional 180.degree. bends in the tubes and provide a tube, or tubes, in a continuous curved, or helical, configuration. However, in order to provide a heat absorption comparable to that of the serpentine arrangement mentioned above and compatible with relatively large installations, the effective height and diameter of such a helical tube would be such that additional serious problems are created. For example, in an installation having a helical tube of a relatively large height, it is difficult to get enough heat to the top portion of the tube and to remove the tube for replacement, repair, or the like. Also, there is a practical limitation to the size of coil which can be accommodated and enclosed in a furnace structure.